This invention relates to the field of signal quality analysis in a digital data acquisition instrument, and more particularly to the field of digital signal quality analysis in a logic analyzer using simultaneous dual-threshold data acquisition.
Some classes of digital data acquisition instruments, e.g., logic analyzers, periodically compare signals under test to a reference threshold in order to determine whether they are in a logical high or a logical low state at each sample time. Some of the more modern of these instruments offer a means for an operator to simultaneously compare the signal under test with two thresholds instead of one. The usual use for such a "dual threshold" capability is to detect signals that are not in either logic state, high or low, but rather in the region of uncertainty between logic levels.
In digital systems it is useful to be able to determine the margin between noise on the data lines and the switching threshold of the circuitry being used. One logic analyzer manufacturer, Gould Test & Measurement Inc., has produced a logic analyzer, the K-105-D, that has optional software which automates a form of noise margin analysis using a single threshold. Acquisitions based on the same trigger definition are taken repeatedly while the single threshold is varied incrementally. When differences in the data start to appear, the effects of noise have been detected and the noise margin can be calculated.
One limitation of this type of noise margin analysis is that the data in the system under test must be repetitive. This is so that, before noise is encountered, the data acquired by successive acquisitions is stable and can serve as a reference. Another limitation of single threshold noise margin analysis is that the trigger location may be one of the first things to be affected when noise is encountered. Once noise causes the trigger location to become affected, the reference for aligning the acquisitions is lost and no further detailed analysis of when and where different levels of noise are occurring is any longer possible. Furthermore, this tendency of the trigger location to be lost as a single threshold is changed also limits the additional signal quality measurements that can be made.
What is desired is a method of performing signal quality analysis that can be used on either repetitive or non-repetitive digital data to identify noise and other signal anomalies, such as slow rise and fall times or marginal pulse widths or amplitudes, in a way that permits such analysis to be continued beyond the variable threshold level where noise or other anomalies are initially encountered.